So for the last 3 years and 58103 miles of tracking I was driving a 2008 Sierra Denali AWD with a Black Bear tune and Airaid MIT with an AEM dry filter. That truck got a lifetime MPG of 13.1. Over the last 25k miles, I averaged 12.8 which was after I put in a Texas Speed custom grind cam.
Since getting my 2016 Sierra Denali 6.2/4WD with 3.23 gears on 12/18 and putting 1065 miles, I'm averaging 16mpg. I use the truck for work. I own and operate a window cleaning business, so about 60% is highway, and the other 40% is city/rural with lots of starts and stops as I go from job to job. I also carry about 400lbs of equipment/water with me in the bed and I don't really have a light foot. I'm quite happy to be getting better MPG and am hoping to get that up to 17 or more average. I've ordered a Magnaflow 12909, and will be getting a tune and CAI soon as well.